This invention relates to handoff control used for a mobile unit either in a CDMA (code division multiple access) mobile communication network or in a combination of the CDMA mobile communication network and a neighboring or contiguous mobile communication network, such as an analog cellular network, which is operable in accordance with other than a CDMA technique.
In the manner described in U.S. Pat. No. 5,295,153 and as will later be described in greater detail, a CDMA mobile communication network has a service area in which a mobile unit is movable and which is divided into a plurality of cells including first and second cells neighboring each other. The CDMA mobile communication network comprises a control station and first and second base stations connected in the first and the second cells to the control station. First and second service frequencies are assigned to the first and the second base stations for traffic communication with the mobile unit while the mobile unit is in the first and the second cells, respectively.
The control station may or may not be situated in the service area. In any event, the control station comprises a control device for detecting necessity of handoff between the first and the second service frequencies and for dealing with the handoff when the necessity is detected.
Like the CDMA mobile communication network, the above-described combination has a common service area in which a mobile unit is movable and which is divided into a plurality of cells including first and second cells neighboring each other. The combination comprises a control station and first and second base stations connected in the first and the second cells to the control station. First and second service frequencies are assigned to the first and the second base stations for traffic communication with the mobile unit while the mobile unit is in the first and the second cells.
The control station may or may not be in the common area. When situated in the common area, the control station may be in a component service area of whichever of the CDMA mobile communication network and the neighboring mobile communication network. In any event, the control station comprises a control device operable in the manner described in the foregoing.
When the combination is taken into consideration with attention directed to the United States patent referenced above, the first cell is in the neighboring mobile communication network and is an urban or umbrella cell. The second cell is in the CDMA mobile communication network and is a rural cell or a microcell. Attention is primarily directed to allocation of frequencies between such cells of different cell sizes. In this connection, the handoff is discussed as regards a cell border between the first and the second cells, namely, as regards a difference between uplink (from the mobile unit to a base station) and downlink (from the base station to the mobile unit) handoff points. In order to use Mobile Assisted Handoff (MAHO) in the CDMA mobile communication network, use is made of a pilot signal or sequence and of a subtraction demodulation process.
In such a conventional mobile communication network and particularly in an analog mobile communication network, it has been the practice to deal with the handoff control as follows. On describing the handoff control, it will be presumed that the mobile unit is currently present in the first cell in traffic communication with the first base station, namely, transmitting an uplink traffic signal to and/or receiving a downlink traffic signal from the first base station, on the first service frequency and is moving towards one of neighboring cells that may be called a destination cell. Furthermore, attention will be directed as a representative to only one of such mobile units which may be concurrently alive in the first cell. Although different frequencies may be assigned to a base station in transmitting the uplink and the downlink traffic signals, such different frequencies will collectively be called a service frequency.
When the mobile unit approaches the cell border between the first cell and the destination cell, an electromagnetic or radio wave of the first service frequency becomes very weak to be objectionable at the mobile unit. In this event, the first service frequency must be handed off or over to a service frequency which is assigned to the destination cell, namely, to the base station placed in the destination cell.
It takes a long time to find which of the neighboring cells is actually the destination cell. During this long time, the traffic signal is received at the mobile unit and the first base station with an objectionable signal quality. Moreover, each base station and consequently the mobile communication unit have a large scale. A great amount of information must be transmitted between the control station and the base stations situated in the neighboring cells.